Transforming the fully-connected structures of hybrid precoders into dynamic partially-connected structures

Partially connected structures for hybrid precoders have called attention because they reduce the number of phase shifters and eliminate the need for radio frequency (RF) adders. There are two categories of partial structures, fixed and dynamic. In fixed structures, each RF chain is connected to an antenna subarray, whereas in dynamic structures, the antenna elements are switchable among the RF chains. Dynamic structures have more freedom degrees and therefore, they offer better performance but in compensation, it is required one switch per antenna and an algorithm to select the switches' positions. This paper extends and comments on the methodology described in a recently published paper. The considered subarrays have a variable size so that the number of switches is decreased, users can receive multiple streams, and the solution for the subarray assignment problem optimization is also extended. Thus, a tool to transform the fully connected structure of a hybrid precoder into dynamic subarray structures is described throughout this paper. Since the number of the switches' positions grows in a factorial manner as the number of RF chains and subarrays increases, exploring brute force is prohibited, therefore an addressed search is necessary. The proposed method explores a few combinations and finds the best among them. However, a better criterion to select these combinations is required. The numerical results evidence that the described tool is more useful in scenarios with a low number of RF chains, low-size subarray, users receiving one single stream, and that the fully-connected analog precoder does not compute any matrix inversion.